Diagnose Engine Stalling Problem

Copyright AA1Car

An engine that stalls repeatedly can be really annoying. So can an engine that stalls for no apparent reason while driving. Stalling problems are often temperature related and more apt to occur during cold weather or when a cold engine is first started.

Problem: Engine stalls when cold immediately after starting

This kind of stalling problem often means the engine is not getting enough fuel and/or too much air. A cold engine needs a fairly rich fuel mixture to start, and to idle smoothly while it warms up. Any of the following could cause or contribute to this kind of stalling problem:

* An engine vacuum leak. Check for loose or broken vacuum hoses, leaks around the intake manifold gasket or throttle body, leaks around the PCV valve and EGR valve.

* A dirty or defective airflow sensor. A sensor that has been contaminated by fuel varnish or dirt will under report airflow and be slow to react to changes in airflow. This can upset the air/fuel mixture causing idle, stalling and hesitation problems. Cleaning the airflow sensor wire with aerosol electronics cleaner can often restore normal operation and cure the problem.

* A defective idle speed control system. Idle speed on a fuel injected engine is controlled by allowing a small amount of air to bypass the throttle. If the idle air bypass circuit is plugged with dirt or fuel varnish, or the solenoid valve is sticking or broken, the engine may not get enough air to idle normally causing it to stall. Cleaning the idle air bypass circuit in the throttle body with aerosol throttle cleaner will often remove the gunk and solve your stalling problem. If a good soaking with cleaner fails to fix the stalling problem, check the wiring connector. It might be loose or corroded. If no wiring faults are found, you may have to replace the idle speed control solenoid.

* A faulty coolant sensor. If the coolant sensor is bad and tells the PCM the engine is colder or warmer than it really is, that can screw up the fuel mixture, too. If the coolant sensor reads colder than normal, or cold all the time, the engine will run rich. This won't cause cold stalling but it can make for a rough idle once the engine warms up, and it kills fuel economy. If the coolant sensor reads warmer than normal, or reads hot all the time, the PCM will lean out the fuel mixture too much, causing the engine to stall when it is cold. See the article on coolant sensors for how to test the sensor. Replacing a defective coolant sensor will cure this cause of stalling.

* A faulty air temperature sensor. This sensor tells the PCM the temperature of the air entering the intake manifold. The PCM needs an accurate input so it can balance the air/fuel mixture properly. Just like a bad coolant sensor, a bad air temperature sensor can upset the fuel mixture causing stalling problems.

* A bad Manifold Absolute Pressure (MAP) sensor. This sensor monitors intake vacuum, which the PCM uses to determine engine load. If the MAP sensor is not reading accurately, the PCM may add too much fuel or not enough, causing the engine to stall. See the article on MAP sensors for how to diagnose this sensor.

* Low engine compression. If your engine has a lot of miles on it and compression is low because the piston rings and/or cylinders are worn, ot it has one or more leaky valves, it may not have enough oomph to keep idling. A compression check will tell you if this is a problem or not, and if it is there's no easy fix other than to overhaul or replace the engine.

* Worn or fouled spark plugs. Ignition misfire can make any engine stall at idle. When the engine is running slowly, there is less momentum to keep it going, so a bad misfire may cause it to stall. If the spark plugs have not been changed in a long time, a new set of plugs and/or plug wires can restore a good hot spark and eliminate the misfire. A weak ignition coil or a faulty crankshaft position sensor may also cause a stalling problem.

* Bad gas. Gasoline that contains too much alcohol (more than 10%), or gasoline that has been contaminated with water or some other substance may not burn well and cause your engine to stall. If the stalling started to occur shortly after your last fill-up, suspect bad gas. The cure is to drain the tank and refill it with fresh gas from another filling station, or just use up the bad fuel (if the engine runs okay at highway speeds), then refill at another station when the tank is near empty.

Problem: Engine stalls when you stop for a traffic light or when idling

A stop light or idle stall often means the engine is not idling fast enough (idle speed too low), or the engine is being lugged down by a load on it created by the air conditioning compressor and/or alternator. It could also mean the fuel mixture is too rich or too lean, causing the engine to run poorly. Possible causes that may contribute to this kind of stalling include:

* A Bad A/C compressor. If the A/C compressor is binding up, possibly due to a lack of lubrication, internal wear or an over-charged A/C system (too much refrigerant), it may be lugging down the engine when it is engaged. If the problem only occurs when the A/C is on, there is an issue with the compressor.

* Unusually high electrical load on the charging system. If the battery is run down and the alternator is working hard to recharge it, the increases load on the engine may pull down the idle rpm to the point where it causes the engine to stall. Check the battery state of charge to see if the battery is run down or failing. If the battery is low, use a battery charger to recharge it, or drive at highways speeds for half an hour or so. If the battery is failing and is not holding a charge, time to buy a new battery.

NOTE: Low voltage can adversely affect the operation of the ignition system and fuel injectors, causing stalling and misfiring. A good charging system should produce about 13.5 to 14.5 volts at idle.

Problem: Engine stalls unexpectedly while driving

Stalls like this are often ignition-related and happen when the engine loses spark. The underlying cause is often a bad crankshaft position sensor, or sometimes a failing ignition coil (if the engine has only one coil). A faulty ignition switch that loses contact intermittently may also cause the engine to suddenly die for no reason.

When this happens, open the hood and check for spark. This can be done by pulling off a plug wire (if the engine has plug wires), and placing the end near the block while a helper cranks the engine. DO NOT hold the wire as it may shock you if the ignition system is working. If you do not see a spark or hear the plug wire snapping when the engine is cranking, the fault is in the ignition system.

If the engine has spark, it may have died due to a loss of fuel pressure. When fuel pumps fail, they usually just quit with little or no warning. The engine usually won't restart and the vehicle has to be towed in for repairs. Listen for a buzz from the vicinity of the fuel tank when the ignition is turned on. No buzz means the fuel pump isn't running. It might just be a blown fuse or a bad relay, but on a high mileage vehicle it's often a bad fuel pump.

Another possibility is a bad PCM (engine computer) relay. The power supply to the PCM is often routed through one or two main power relays. If one of these relays loses contact momentarily, it's like pulling the plug on the PCM. The PCM shuts down and turns off the ignition and fuel injectors, causing the engine to stall. One way to see if this is a possibility is to switch or replace the PCM power relay(s). If the problem goes away, the cause was a bad relay. If it continues, the fault is something else (possible a wiring fault in the PCM relay or PCM power circuit).

Yet another possibility is low system voltage, loss of voltage, or overcharging. The PCM and other control modules require a steady 12 volts to operate correctly. If the supply voltage suddenly drops below 9 volts, or surges about 16 volts, or cuts out, the PCM may temporarily kill the injectors or ignition circuit. The underlying cause may be an intermittent short somewhere in the electrical system or charging system that causes a momentary drop or surge in voltage. These can be very difficult to find, and often require hooking up a scan tool that can capture snapshot data when the stall occurs. By looking at the data, a technician can see the chain of events that caused the stall, and hopefully identify, isolate and repair the fault.
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